Solid-state lighting (SSL) is an energy-saving lighting technology. Light-emitting diodes (LED) and organic light-emitting diodes (OLED) are two main branches of SSL technology that are evolving rapidly in recent years (Optoelectronics Industry Development Association, The Promise of Solid State Lighting for General Illumination; Washington, DC, 2001). Utilizing semiconductor materials, SSL devices convert electricity to light much more effectively than conventional lighting sources. Additionally, they contribute to the reduction of green house gases and offer a cleaner environment (Uchida et al., Optical Energy, 44, 124003-1 (2005); Taguchi, Proc. SPIE 5530, 7-16 (2004); Taguchi, J. Light Visual Environ. 27, 131 (2003)). Solar powered LEDs can make direct use of renewable energy for our society's lighting needs (See, e.g., Xiao et al., Inorg. Chem. 42, 107-111 (2003)). It has been estimated that an approximate 29% reduction of US and 50% reduction of global energy consumption for lighting could be reached by 2020 as a result of solid-state lighting applications (Navigant Consulting Inc., Energy Savings Potential of Solid State Lighting in General Illumination Applications, Washington, DC (2003); Bergh et al., Phys. Today 54, 42-47 (2001)).
White-light LEDs, which have great potential for general lighting applications, are produced by either mixing of red, green and blue (RGB) LEDs, or by phosphor conversion, in which white light is generated by coating a blue or near-UV LED with a yellow or multichromatic phosphor (See, e.g., Uchida et al., Optical Energy, 44, 124003-1 (2005)). However, both processes are associated with complex issues and schemes, and significant reduction of device efficiency due to problems such as self absorption, relatively low light capture efficiency of phosphors or non-radiative carrier losses (See, e.g., Mueller et al., Nano Lett. 5, 1039-1044 (2005); Yang et al., Chem. Mater., 17 3883-3888 (2005)).
A recent discovery shows that ultra-small (magic-sized) CdSe nanocrystals (NCs) give rise to a broad (white-light) emission covering the entire visible spectrum (Bowers et al., J. Am. Chem. Soc., 127, 1537815379 (2005)), as a direct result of very high surface-to-volume ratio and thus, a significantly larger number of mid-gap surface sites. While they may offer some advantages over other phosphors, weak correlations among the nanocrystals limit their uses as direct white-light emitting diodes, because of the difficulties in achieving high conductivity and mobility required by a LED device. A more desirable and potentially economical approach is to develop semiconductor bulk materials that can be used as direct white-light emitting diodes.